This invention relates to an information recording medium which is applicable to an optical memory element, an optical disk, an optical card, an optical document and image filing system, and the like.
Heretofore, metallic films of Al, Cr, Cu, Au, Ni, Ti and Bi, reported in The Industrial Daily News dated Apr. 14, 1987, and films of chalcogens or chalcogenated compounds such as Te and Te compounds, disclosed in M. Terao, et al.; J. Appl. Phys., 50, 6881 (1979), have been known as a write-once type recording medium of a heat mode memory type (a melting and evaporation type). However, such metallic films have a poor laser beam absorption efficiency. It is therefore necessary to apply a strong laser beam (100 mW) to the metallic film memory medium for a long time (1 .mu.sec) when recording is performed. In particular, when recording is performed at high speed, the metallic film memory medium does not provide sufficient sensitivity for recording. Furthermore, when bits are formed on the metallic film memory medium, a protuberance is co-formed around each bit, and such protuberances increase noise when information is reproduced. In addition to the above, a vacuum evaporated film of a chalcogenated compound has the shortcoming of being readily deteriorated by heat and humidity.
Films of chalcogen compounds such as Te and Te compounds have been known as being usable as information recording media of a phase transition type [S. R. Ovshinsky; Appl. Phy's. Lett., 18, 254 (1971)]. However, the above films show only a small change in reflectivity of about 10% during recording, so that poor contrast is obtained.
As an optical memory medium of a bubble mode type, a sputtered film of In-CH.sub.4 -O.sub.2 has been known [Takeoka et al; Optical Memory Symposium '85, 39 (1985)]. The constituent element In, however, is readily oxidized. In addition, In or In.sub.2 O.sub.3 and an organic component exist separately in the film as a cluster, so that the film becomes unstable in the course of preservation for a long time or under highly humid conditions. Further, since the protuberance of the bubbles formed on the memory medium is as small as 1000 .ANG., high contrast cannot be obtained.
Further chalcogenated-compound-deposited films utilizing a phase transition between one crystal phase and another crystal phase or between a crystal phase and an amorphous phase are known as information recording media which are capable of recording and reproducing information and erasing recorded information. Unlike magneto-optical recording media, the phase transition type information recording media are capable of overwriting by the application of a single beam, and therefore an optical system for driving the recording medium is simple in mechanism. Thus, recently research and development activities are actively directed to the phase transition type information recording medium. Representative examples of the materials for use in the phase transition type information recording media are Ge-Te, Ge-Te-Sb, Ge-Te-S, Ge-Se-S, Ge-Se-Sb, Ge-As-Se, In-Te, Se-Te and Se-As as disclosed in U.S. Pat. No. 3,530,441.
For improvement of the stability and high speed crystallization of the material for use in the recording media, Au-added Ge-Te is proposed in Japanese Laid-Open Patent Application 61-219692, Sn-and-Au-added Ge-Te in Japanese Laid-Open Patent Application 61-270190, and Pb-added Ge-Te in Japanese Laid-Open Patent Application 62-19490.
Furthermore, a Ge-Te-Se-Sb having a particular composition is proposed in an attempt of improvement of repeated recording and erasing performance in Japanese Laid-Open Patent Application 62-73438.
However, none of the above proposed phase transition type information recording media can satisfy all the necessary properties for rewritable information recording media. In particular, recording sensitivity, erasing sensitivity, the erasing ratio at the time of overwriting, and the life of recorded portions and unrecorded portions are still insufficient for use in practice, and these are very important problems to be solved.
Of the above various problems, the extension of the life of recorded portions and unrecorded portions is the most important target to be cleared in both write-once type information recording media and rewritable type recording media.
In order to clear this target, an information recording medium comprising a transparent matrix and a light absorbing material dispersed in the transparent matrix is proposed.
As the materials for the matrix, oxides of Si, Al, Ti and Mg are disclosed in Japanese Laid-Open Patent Application 57-208648. Furthermore, in Japanese Laid-Open Patent Application 63-173240, metallic oxides, metallic nitrides, metallic sulfides, and metallic carbonates having small heat conductivities, such as SiO.sub.2, SiO, Si.sub.3 N.sub.4, TiO.sub.2, ZnS, ZnO, Al.sub.2 O.sub.3, AlN, MgO, GeO, SiC, ZrO.sub.2 and Nb.sub.2 O.sub.5, are proposed. However, the matrixes made of these inorganic materials have the drawbacks that the growth of crystal nuclei or crystals occurs during repeated information recording and erasing cycles, resulting in imperfect erasing and reduction of the C/N ratio caused by grain boundary noise.
Furthermore, organic materials for use in such matrixes as proposed in Japanese Laid-Open Patent Applications 60-124038, 63-205832 and 63-206921. In these references, a method of fabricating information recording media by simultaneous sputtering of a metal such as Te and a heat resistant resin is described. In Technical Report CPM83-58 published by The Institute of Electronics and Communication Engineers, a method of fabricating information recording media by reactive sputtering of Te and CH.sub.4 is described. However, both of the above methods have the shortcoming that it is extremely difficult to control the particle size and dispersing state of the materials to be dispersed, such as Te. When an organic matrix is prepared by sputtering, the pressure of the reaction atmosphere is about 10.sup.-2 Torr, so that the coagulation of the dispersed material is inevitable and it is extremely difficult to disperse the material in a finely-divided state.
In the case where only organic metallic compounds are employed as starting materials for the preparation of a recording layer of an information recording medium by a glow discharging decomposition method, it is almost impossible to prepare a recording layer including a transition metal because stable organic metallic compounds containing transition metals are rarely available.